1. Field of the Invention
The present invention relates to a method and apparatus for surface-hardening steel balls produced of hardened steel.
2. Description of the Related Art
Japanese Patent Laid-Open Application No. 195069/1993 describes approaches for surface hardening steel balls (e.g. after quenching and tempering) used as bearing balls which are produced of a high carbon chromium bearing steel (JIS G4805) represented by SUJ2 or a martensitic stainless steel (JIS G4303) represented by SUS440C. The surface hardened steel balls have a surface layer with greater residual compressive stress and hardness than untreated steel balls.
Referring to FIG. 8, Japanese Patent Publication No. 12813/1989 describes steel balls 101 charged into a regular octagonal steel barrel 100 through a port 103 to, at most, about two-thirds the inside capacity of barrel 100. The barrel 100 is rotated in the direction of arrow P about a center axis 104 supported on a frame 102. Upon rotating barrel 100, the steel balls 101 move upwardly, and subsequently fall in the direction of arrow Q. The falling steel balls strike against the steel balls 101 lying below and against the inner wall of barrel 100. By continuously repeating this operation, the entire surface of the steel balls 101 are surface hardened. The finished-steel balls are discharged through port 103.
The method described above, however, has the following disadvantages:
(1) If the barrel rotates at an excessively high speed, the steel balls are held against the inner wall of the barrel by a centrifugal force, and thus will not strike against the inner wall. Therefore, it becomes necessary to decrease the revolution number of the barrel to 80 rpm or lower. PA1 (2) The smaller the diameter of the steel ball, the smaller the striking force per unit time. For example, in the case of a 25 mm steel ball, the net weight of the ball is 0.62 N (63.57 gf), whereas in the case of a 3 mm steel ball, the net weight of the ball is 0.001 N (0.11 gf). PA1 (3) To surface harden 3 mm diameter steel balls in a predetermined time, it is necessary to increase the height from which the steel ball is dropped. This, however, requires a larger size barrel, and correspondingly a larger size surface-hardening apparatus, resulting in a lower operation performance. PA1 (4) Recently, steel balls for ball bearings used in e.g., automotive transmissions are required to have a long service life and to remain usable even if foreign substances become introduced in the lubricating oil. The steel balls employed for such service must have a greater residual compressive stress and hardness in the surface layer.
At this lower speed, the frequency at which the steel balls strike against each other or against the inner wall is less, such that a longer time is needed to achieve a desired surface residual compressive stress and hardness.
Since the revolution number of the barrel is limited for the reason described above (1), a much longer time is required for surface-hardening small-diameter, e.g., 3 mm steel balls.
Consequently it is extremely difficult to surface harden very small steels balls measuring 1 to 3 mm in diameter.
To produce steels balls (particularly those of smaller diameter) having surfaces with the required residual compressive stress and hardness, it becomes necessary to increase the barrel size and/or to prolong the treatment time.